Recently, a power device is made of silicon carbide (i.e., SiC) in order to obtain high electric field breakdown strength. Since the SiC semiconductor device has the high electric field breakdown strength, the device can control large current. Thus, the device can be used for various fields. For example, in an automobile industry field, the device may be used for controlling a motor in a hybrid vehicle (i.e., HV), an electric vehicle (i.e., EV), a fuel cell vehicle (FCV) and the like.
In the SiC semiconductor device, in order to flow a large current, it is effective to increase a channel density. In a silicon transistor, a trench gate type MOSFET can flow the large current. The SiC semiconductor device can have the trench gate structure. For example, in JP-A-2009-231545 corresponding to US 2009/0236612, the trench gate type SiC semiconductor device is described.
However, when the trench gate type MOSFET is formed in a SiC substrate, the following difficulties arise.
When a gate oxide film is formed, a damage and/or an impurity remain in a surface portion of the SiC substrate. The damage and the impurity are introduced in the substrate during a previous process. Specifically, the damage caused in a trench etching process may remain on a sidewall of a trench formed in the trench etching process. When the damage remains on the sidewall, current may be leaked through the damage. Thus, to remove the damage, a hydrogen etching process and/or a sacrifice oxidation process may be performed.
However, an oxidation rate in a high impurity concentration region is high so that the high impurity concentration region is oxidized rapidly. Accordingly, when the damage is removed, and the sacrifice oxidation process is performed until the damage is completely removed, a surface concentration in a source region having a high impurity concentration and formed at a shoulder of the trench (i.e., a corner of an opening side of the trench) is not maintained to be high concentration. Specifically, the concentration of a contact portion with the source electrode is not maintained to be high concentration. Thus, a contact resistance increases.
For example, in order to increase the concentration of the source region to be higher than the contact portion, a part of the source region disposed at a deep position is formed by an ion implantation method, and another part of the source region disposed at a shallow position is formed by an ion implantation method with using phosphorous (i.e., P), so that the concentration of the source region at the shallow position is higher than the source region at the deep position. For example, the concentration of the source region at the shallow position is 1×1020 cm−3. In this manufacturing process, the contact portion of the source region at the shallow position has an impurity concentration higher than the contact portion of the source region at the deep position, so that the contact resistance is reduced. However, the part of the source region at the shallow position is oxidized by the rapid oxidization effect, and after that, the part of the source region at the shallow position is removed when the sacrifice oxidation film is removed. Thus, the part of the source region, which has the high impurity concentration, may be removed. Thus, the contact portion between the source region and the source electrode does not provide the high impurity concentration portion, and therefore, the contact resistance increases. Further, the thickness of the source region is thinned, so that the cross sectional area of a current path is reduced. Accordingly, the resistance is further reduced.
Similar difficulties may arise not only in a case where the source region is formed by a method with using two different ions such as nitrogen and phosphorus but also in a case where the source region is formed by a method with using the same ion such that the part of the source region at the shallow position has the impurity concentration higher than the part of the source region at the deep position. Further, not only in a case of the trench gate type MOSFET but also in a case of a trench gate type IGBT, similar difficulties arise. Here, the damage is removed by the sacrifice oxidation method. Alternatively, when the damage is removed by a hydrogen etching method, the high impurity concentration layer is rapidly etched. Therefore, similar difficulties arise.